N-methylpyrrolidinium-based ionic liquid, with fluorine function group, N-(2-fluoroethyl) -N-methylpyrroli-dinium bis(trifluoromethanesulfonyl)imide is proposed as the co-solvent for Si anode. Scanning electron microscope, X-ray photoelectron spectroscopy as well as the impedance measurement is conducted to investigate its effect on the solid electrolyte interface formation. Cycling tests show that it can much better improve the cycling stability of Si than fluoroethylene carbonate or F-free ionic liquid analogue.